As products utilizing a piezoelectric ceramic, hitherto, there are piezoelectric sensors, filters, piezoelectric resonators, ultrasonic transducers, ultrasonic motors, and the like, for example. The piezoelectric sensors are used as a shock sensor, an acceleration sensor, or a knock sensor for automobile use, and particularly in recent years, the piezoelectric sensors are studied as a pressure sensor for directly detecting a pressure in a cylinder to optimize a timing of fuel injection from a fuel injector for the purpose of an improvement in fuel economy and reduction in exhaust gas (HC, NOx) of an automobile engine.
Conventionally, as a piezoelectric ceramic, a PZT (lead zirconate titanate)-based material or a PT (lead titanate)-based material, which has a high piezoelectric property, for example, a large piezoelectric coefficient d, has been used. However, it is pointed out that since the PZT-based material and the PT-based material respectively contain lead in an amount of about 60% by mass, these materials are at risk of eluting lead due to acid rain to invite environmental pollution. Thus, high expectations are placed on a lead-free piezoelectric material.
Further, since the PZT-based material or the PT-based material has a Curie temperature Tc of about 200° C. to about 300° C., the piezoelectric coefficient d is deteriorated in use under an elevated temperature of about 200° C. and the variation between the piezoelectric coefficient d at room temperature and the piezoelectric coefficient d at 200° C. is large, and therefore there are large constraints on its applications. For example, in the case of being used as a pressure sensor, if the piezoelectric coefficient d is deteriorated due to change with time, an output voltage varies even under the same pressure, and if the variation between the piezoelectric coefficient d at room temperature and the piezoelectric coefficient d at 200° C. is large, it becomes difficult to calculate an accurate pressure from the output voltage because the relationship between the pressure and the output voltage is not linear. Further, in order to obtain pressure sensor characteristics which are stable even under an elevated temperature of 200° C., a method using a single crystal such as langasite or quartz has also been investigated. However, when the single crystal is used, there is a problem that the piezoelectric coefficient d is small and chipping easily occurs and the single crystal is easily broken in processing the single crystal, and furthermore there is a problem that the production cost of the single crystal is extremely high.
Accordingly, a material which is predominantly composed of a bismuth layered compound is proposed as a piezoelectric ceramic not containing lead (for example, Patent Document 1). Many of the piezoelectric ceramics predominantly composed of a bismuth layered compound have a Curie temperature of about 400° C. or higher, and such a piezoelectric ceramic has high heat resistance and has the potential to be applied to a sensor which is used in an environment which is exposed to elevated temperatures, for example, in an engine room.    Patent Document 1: Japanese Unexamined Patent Publication No. 2002-167276